The function of a light receiver is to generate an electrical signal from incident reception light. The detection sensitivity of simple photodiodes is not sufficient in many applications. In an avalanche photodiode (APD), the incident light triggers a controlled avalanche breakdown (avalanche effect). This multiplies the charge carriers generated by incident photons, and a photo current is produced, which is proportional to the light reception level, but significantly larger than in a simple PIN diode. In a so-called Geiger mode, the avalanche photodiode is biased above the breakdown voltage so that even a single charge carrier generated by a single photon can trigger an avalanche, which subsequently recruits all available charge carriers due to the strong field. Hence, the avalanche diode counts individual events like a Geiger counter from which the name is derived. Geiger mode avalanche photodiodes are also called SPAD (Single Photon Avalanche Diode).
The high radiation sensitivity of SPADs is used in a number of applications. These include medical technology like CT, MRI, or blood tests, optical measuring technology like spectroscopy, distance measurement and three-dimensional imaging, radiation detection in nuclear physics, or uses in telescopes for astrophysics.
Geiger APDs or SPADs thus are very fast, highly sensitive photodiodes on a semiconductor basis. One drawback of the high sensitivity is that not only a measurement photon, but also a weak interference event from ambient light, optical cross talk or dark noise may trigger the avalanche breakdown. The interference event contributes to the measurement signal with the same relatively strong signal as the received measurement light and is indistinguishable within the signal. The avalanche diode subsequently is insensitive for a dead time of about 5 to 100 ns and is unavailable for further measurements during that time. It is therefore common to interconnect and statistically evaluate multiple SPADs. Nevertheless, large light power leads to saturation effects in individual pixels or entire regions. In particular when a large dynamic range is to be covered, important information about the reception light may be lost.
The breakdown voltage is the minimal bias voltage necessary for maintaining the desired Geiger mode for a SPAD. Strictly speaking, however, the detection efficiency and the gain are still zero at this limit. Only when the bias voltage exceeds the breakdown voltage are incident photons converted into corresponding Geiger current pulses. In case of ideal photon detection efficiency (PDE) of 100%, each incident photon would trigger a Geiger current pulse. This is not completely possible in practice. However, the PDE can be influenced by the magnitude of the applied bias voltage.
In order to set the operating point of the SPADs and accordingly their triggering sensitivity via a bias voltage provided externally, the anode-side and cathode-side connections of the individual SPAD cells of the light receiver are directly accessed from the outside. Thus, all SPADs are operated with a common bias voltage. Instead of the bias voltage, sometimes only the overvoltage is considered, i.e. the difference between bias voltage and breakdown voltage. The triggering probability increases with the overvoltage. In practice, there is a reasonable upper limit, because the triggering probability saturates at higher overvoltages, while undesired noise components increase disproportionately. The operating point set by means of the overvoltage enables a certain adaptation of the light receiver, but does not account for many situations with greatly varying or temporally fluctuating reception light conditions.
WO 2011/117309 A2 proposes to provide, in addition to the anode and cathode for providing the bias voltage, a third electrode on the SPAD detector, the third electrode being used for a capacitively coupled output of the Geiger current. This is to prevent that the readout is delayed by switching elements of the bias voltage. However, this does not improve the adaption of the light receiver to ambient or application conditions.